Base Defense is a single/cooperative mod based on horde-style and RPG systems. It's awesome, you should try it ;)
The war of good and evil was interrupted by alien invasion. Demons ran off, angels got back to heaven. Only humans left, surrounded by aliens. Here come 3 characters called defenders. They send all the humans to another realm, which is causing the merger of past and actual times. All events in the game are happening after the merger.